Receiver/transmitter co-calibration of voltage levels in pulse amplitude modulation links

ABSTRACT

A driver circuit of a PAM-N transmitting device transmits a PAM-N signal via a communication channel, wherein N is greater than 2, and the PAM-N signal has N signal levels corresponding to N symbols. A PAM-N receiving device receives the PAM-N signal. The PAM-N receiving device generates distortion information indicative of a level of distortion corresponding to inequalities in voltage differences between the N signal levels. The PAM-N receiving device transmits to the PAM-N transmitting device the distortion information indicative of the level of the distortion. The PAM-N transmitting device receives the distortion information. The PAM-N transmitting device adjusts one or more drive strength parameters of the driver circuit of the PAM-N transmitting device based on the distortion information.

BACKGROUND

In high speed data transmission systems, pulse amplitude modulation(PAM) signaling is used to carry data from a transmitting device to areceiving device. While a 2-level PAM (PAM-2) signaling system isinherently linear, a multi-level PAM signaling system (PAM-N) with morethan 2 levels can suffer from non-linearity distortion. Differentcomponents within the transmitter and receiver can introduce non-uniformdistribution of PAM target levels in the PAM signal. These unwantedvoltage offsets from the multi-PAM levels can affect the ability of thereceiver to correctly recover data from the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the embodiments herein can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings.

Figure (FIG. 1 is an illustration of a system with an examplereceiver/transmitter calibration architecture, according to anembodiment of the present disclosure.

FIG. 2 is an illustration of an example receiver interface circuit ofthe PAM-N receiving device of FIG. 1 according to an embodiment of thepresent disclosure.

FIG. 3 is an illustration of example waveforms depicting an idea PAM-Neye diagram and a PAM-N eye diagram with level distortion, according toan embodiment of the present disclosure.

FIG. 4 is an illustration of an example driver circuit of the PAM-Ntransmitting device of FIG. 1, according to an embodiment of the presentdisclosure.

FIG. 5 is an illustration of an example distortion detection circuit ofthe PAM-N receiving device of FIG. 1, according to an embodiment of thepresent disclosure.

FIG. 6 illustrates an example method of detecting distortion in a PAM-Nsignal based on estimation of intersymbol interference (ISI), accordingto an embodiment of the present disclosure.

FIG. 7 illustrates an example method of detecting distortion in a PAM-Nsignal based on decision feedback equalizer (DFE) error leveladaptation, according to an embodiment of the present disclosure.

FIG. 8 is an example flowchart illustrating a method ofreceiver/transmitter co-calibration of voltage levels in PAM-N links,according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to several embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying figures. It is noted that wherever practicable similar orlike reference numbers may be used in the figures and may indicatesimilar or like functionality. The figures depict embodiments of thepresent disclosure for purposes of illustration only. One skilled in theart will readily recognize from the following description thatalternative embodiments of the structures and methods illustrated hereinmay be employed without departing from the principles, or benefitstouted, of the disclosure described herein.

An embodiment of the present disclosure relates to a PAM-N receivingdevice, including a receiver interface circuit to receive a PAM-N signalfrom a PAM-N transmitting device via a communication channel, wherein Nis greater than 2 and the PAM-N signal has N signal levels for Nsymbols. A circuit generates distortion information indicative of alevel of distortion corresponding to inequalities in voltage differencesbetween the N signal levels. A driver circuit transmits the distortioninformation indicative of the level of the distortion to the PAM-Ntransmitting device.

In one embodiment, the PAM-N receiving device further includes adecision feedback equalizer, wherein the distortion information isgenerated by determining a first threshold error level used to generateerror information for adaptation of tap weights of the decision feedbackequalizer when targeting a first symbol of the N symbols. A secondthreshold error level used to generate error information for adaptationof the tap weights of the decision feedback equalizer when targeting asecond symbol of the N symbols is determined, wherein the distortioninformation is generated from the first threshold error level and thesecond threshold error level.

In one embodiment, the distortion information is generated byidentifying a first amount of ISI during transitions from a first symbolof the N symbols. A second amount of ISI is identified duringtransitions from a second symbol of the N symbols, wherein thedistortion information is generated from the first amount of ISI and thesecond amount of ISI.

In one embodiment, the circuit identifies the first amount of ISI duringtransitions from the first symbol to a third symbol of the N symbols.The second amount of ISI is identified during transitions from thesecond symbol to a fourth symbol of the N symbols.

In one embodiment, the circuit includes an eye scanning circuit todetect vertical eye openings for the PAM-N signal, wherein thedistortion information is generated from the vertical eye openings.

In one embodiment, the circuit detects when a predetermined pattern ispresent in the PAM-N signal and generates the distortion informationresponsive to the predetermined pattern being present.

In one embodiment, the receiver interface circuit of the PAM-N receivingdevice includes an analog front end circuit that applies analog signalconditioning to the PAM-N signal. The circuit generates the distortioninformation indicative of the level of the distortion corresponding tothe inequalities in the voltage differences between the N signal levelsin the PAM-N signal at an output of the analog front end circuit.

In one embodiment, a PAM-N transmitting device includes a driver circuitto transmit a PAM-N signal via a communication channel to a PAM-Nreceiving device, wherein N is greater than 2, and the PAM-N signal hasN signal levels corresponding to N symbols. A receiver interface circuitreceives distortion information indicating a level of distortioncorresponding to inequalities in voltage differences between the Nsignal levels at the PAM-N receiving device. A driver control circuitadjusts one or more drive strength parameters of the driver circuitbased on the distortion information.

In one embodiment, the driver circuit of the PAM-N transmitting devicegenerates the PAM-N signal using a predetermined pattern.

In one embodiment, the driver circuit of the PAM-N transmitting devicegenerates the PAM-N signal using a pseudorandom or scrambled pattern.

In one embodiment, the driver circuit of the PAM-N transmitting deviceincludes a first driver corresponding to a first symbol bit and a seconddriver corresponding to a second symbol bit, wherein the driver controlcircuit adjusts the one or more drive strength parameters based on thedistortion information such that a ratio of a first drive strength ofthe first driver to a second drive strength of the second driver isadjusted.

In one embodiment, the one or more drive strength parameters of thedriver circuit of the PAM-N transmitting device are adjusted to minimizethe inequalities in the voltage differences.

In one embodiment, the driver control circuit adjusts the one or moredrive strength parameters to match a peak power constraint of the drivercircuit.

Receiver/Transmitter Co-Calibration Architecture for a PAM-N Link

FIG. 1 is an illustration of a system having a receiver/transmittercalibration architecture 100, according to an embodiment of the presentdisclosure. The architecture 100 includes a PAM-N transmitting device104 and a PAM-N receiving device 154. The PAM-N transmitting device 104may be an integrated circuit (IC) while the PAM-N receiving device 154may be a different IC. Pulse-amplitude modulation (PAM), may be a formof signal modulation where a data symbol 124 is encoded in the amplitudeof a series of signal pulses by the PAM-N transmitting device 104. It isan analog pulse modulation scheme in which the amplitudes of a train ofcarrier pulses are varied according to the sample value of the datasymbol 124.

Demodulation is performed by detecting the amplitude level of the PAM-Nsignal 132 at every single period. PAM-N refers to PAM with N signallevels. For example, some Ethernet standards use five-level PAMmodulation (PAM-5) and some use PAM with 16 discrete levels (PAM-16).

The PAM-N transmitting device 104 includes a driver circuit 128 thatencodes multi-bit data symbols 124 into a PAM-N signal 132 and transmitsthe PAM-N signal 132 via a communication channel 140 over datatransmission wires 148 to the PAM-N receiving device 154, wherein N isgreater than 2, and the PAM-N signal 132 has N signal levelscorresponding to N types of data symbols. In one embodiment, the PAM-Nsignal 132 is a PAM-4 signal. The PAM-N signal 132 can be transmitted asa differential signal over the data transmission wires 148 (only oneline is shown in FIG. 1, but the line can represent a pair of wires148). In other embodiments the PAM-N signal 132 can be a single endedsignal transmitted over a single ended wire. In other embodiments, thePAM-N signal 132 may use different coding schemes in which multiple bitsare encoded and transmitted over multiple lines to trade-off betweendifferential and single-ended signaling.

The PAM-N transmitting device 104 includes a receiver interface circuit108 to receive distortion information 162, via the communication channel140 over data transmission wires 144. The distortion information 162indicates a level of non-linear distortion caused by inequalities involtage differences between the N signal levels at the PAM-N receivingdevice 154. The PAM-N transmitting device 104 includes a driver controlcircuit 116 that receives the distortion information 162 and generatesone or more drive strength control signals 120 for adjusting one or moredrive strength parameters of the driver circuit 128 based on thedistortion information 162.

The PAM-N receiving device 154 includes a receiver interface circuit 172to receive the PAM-N signal 132 from the PAM-N transmitting device 104via the communication channel 140. The receiver interface circuit 172can be an analog front end (AFE) circuit that applies analog signalconditioning to the PAM-N signal 132 to generate a PAM-N signal 176 atthe output of the receiver interface circuit 172. An example of thereceiver interface circuit 172 will be described in detail below withreference to FIG. 2

The PAM-N signal 176 at the output of the receiver interface circuit 172is used as input to a decision circuit 180 that makes a decision on thedata symbols 184 encoded in the PAM-N signal 176. The decision circuit180 can include a data slicer circuit that compares samples of the PAM-Nsignal 176 to one or more decision thresholds. In an alternativeembodiment, an analog to digital converter (ADC) (not shown) may convertanalog voltage samples of the PAM-N signal 176 into digital samples. Thedigital samples are then digitally processed by a digital signalprocessor to identify data symbols 184.

The PAM-N receiving device 154 includes a distortion detection circuit158 to detect non-linear distortion caused by inequalities in voltagedifferences between the N signal levels of the PAM-N signal 176, and togenerate distortion information 162 indicative of a level of thedistortion. The distortion can be detected in the PAM-N signal 176 atthe output of the interface circuit 172, which is the input to thedecision circuit 180. In embodiments that include an ADC (not shown),the distortion can be detected in the PAM-N signal at the output of theinterface circuit 172 which is the input to the ADC.

The PAM-N receiving device 154 includes a driver circuit 166 to transmitthe distortion information 162 indicative of a level of the distortionto the PAM-N transmitting device 104. The distortion information 162 maybe transmitted in packets reserved for physical layer communications.The PAM-N transmitting device 104 uses the distortion information 162 toadjust drive strength parameters of the driver 128. The drive strengthparameters are adjusted until the inequalities in voltage differencesbetween the N signal levels of the PAM-N signal are minimized oreliminated. Thus, distortion introduced by the receiver interface 172can be accounted for by pre-adjusting the drive strength of the driver128 at the transmitting device 104.

Receiver Interface Circuit of the PAM-N Receiving Device

FIG. 2 is an illustration of the example receiver interface circuit 172of the PAM-N receiving device 154 of FIG. 1, according to an embodimentof the present disclosure. The receiver interface circuit 172 includesan analog front end (AFE) circuit 200 that applies analog signalconditioning to the PAM-N signal 132. The components of the AFE circuit200 may operate in a non-linear manner which results in inequalities involtage differences between the N signal levels in the PAM-N signal. Thedistortion detection circuit 158, illustrated with reference to FIG. 1above, detects distortion corresponding to inequalities in the voltagedifferences between the N signal levels in the PAM-N signal 176 at anoutput of the AFE circuit 200.

The AFE 200 portion of the receiver 172 applies analog signal processingto the PAM-N Signal 132 to generate PAM-N Signal 176 at an output nodeof the AFE 200. The AFE 200 may include a variable gain amplifier 204that has a variable gain that amplifies the PAM-N signal 132 to generatethe PAM-N signal 176.

The AFE 200 may include an offset adjustment circuit 208 that corrects adirect current (DC) voltage offset of the various stages of AFE 200 togenerate the suitable PAM-N signal 176 for the following stage. The AFE200 may include a continuous time linear equalizer (CTLE) circuit 212that applies linear equalization to the PAM-N signal 132 to generate thePAM-N signal 176.

Different configurations of the variable gain amplifier 204, the offsetadjustment circuit 208, and the CTLE 212 are possible and the CTLE maybe placed ahead of the other two components or at the end in the signalprocessing flow. In other embodiments, the AFE 200 may also have othercircuits such as discrete- or continuous-time feed forward equalization(FFE) components.

Ideal PAM-N Eye and PAM-N Eye with Level Distortion

FIG. 3 is an illustration of example waveforms depicting an ideal PAM-Neye diagram 300 and a PAM-N eye diagram 350 with level distortion,according to an embodiment of the present disclosure. The PAM-N eyediagram 300 represents an oscilloscope display in which the PAM-N signal176 from the receiver interface circuit 172 is repetitively sampled andapplied to the vertical axis (signal levels 304), while the data rate isused to trigger the horizontal sweep (time axis). It is a synchronizedsuperposition of different realizations of the signal of interest (e.g.the PAM-N signal 176) viewed within a particular signaling interval.

The ideal PAM-N eye 300 plots the four different analog voltage levels304 for the four symbol values 00, 01, 10, and 11 of the signal 176. Thethree level differences 308 in the analog voltage levels 304 (after theyrise/fall and have settled on their steady state voltage levels) arerepresented by 308A, 308B, and 308C. Difference 308A represents thevoltage difference between the analog voltage levels for symbol values11 and 10. Difference 308B represents the voltage difference between theanalog voltage levels for symbol values 10 and 01. Difference 308Crepresents the voltage difference between the analog voltage levels forsymbol values 01 and 00. Example analog voltage levels for the signallevels 304 are illustrated and described in detail below with referenceto FIG. 6.

In an ideal situation, the level differences 308A, 308B and 308C equaleach other. As shown in the ideal PAM-N eye 300, level difference 308A,level difference 308B and level difference 308C are all equal to 2 V.However, in practice, the driver 128 and/or the analog front end 200(and also the channel 140 itself) may operate in a non-linear manner,which causes these level differences 308 to be different from eachother. This distortion is illustrated in the PAM-N eye with leveldistortion 350.

Nonlinearity distortion in the analog voltage level differences 358between the N signal levels 354 is illustrated in the PAM-N eye diagramwith level distortion 350. The PAM-N eye diagram 350 plots the fourdifferent analog voltage levels 354 for the four symbol values 00, 01,10, and 11 of the signal 176. The three level differences 358 in theanalog voltage levels 354 are represented by 358A, 358B, and 358C.Difference 358A represents the voltage difference between the analogvoltage levels for symbol values 11 and 10. In this particular case,difference 358A is less than difference 358B, which represents thevoltage difference between the analog voltage levels for symbol values10 and 01. Difference 358C represents the voltage difference between theanalog voltage levels for values 01 and 00 and is less than difference358B. The three level differences 358 are not equal to each othercausing distortion in the PAM-N eye 350.

In one embodiment, the receiving device 154 provides distortioninformation 162 indicative of a level of the distortion corresponding tothese inequalities in the voltage differences 358 between the N signallevels in the PAM-N signal 176. The transmitting device 104 can thenadjust the drive strength parameters of the driver 128 to minimize thisdistortion such that the voltages differences 358 are closer to theideal voltage differences 308.

Driver Circuit of the PAM-N Transmitting Device

FIG. 4 is an illustration of an example driver circuit 128 of the PAM-Ntransmitting device 104 of FIG. 1, according to an embodiment of thepresent disclosure. The driver circuit 128 converts bits of a datasymbol into a voltage waveform that can be propagated down the channel140.

In the case of a PAM-4 driver, the driver circuit 128 may include twodifferential drivers 400 and 404. The value of N equals 4 in the exampledriver circuit illustrated in FIG. 4. The driver circuit may besimilarly designed for larger values of N using a larger number ofdifferential drivers similar to 400 and 404. The two differential inputterminals to the differential driver 400 are 416 and 420. The inputterminals 416 and 420 carry signals representing the most significantbit (MSB) 432 in a two-bit data symbol. The MSB 432 is the bit positionin the 2-bit binary number, e.g., 01, having the greater weight. Thedrive strength control signal 120 a from the driver control circuit 116of FIG. 1 adjusts a drive strength parameter affecting the drivestrength of the differential driver 400. The input terminals 424 and 428carry signals representing the least significant bit (LSB) 436 in thetwo-bit data symbol. The LSB 436 is the bit position in the 2-bit binarynumber, e.g., 01, having the lesser weight. The drive strength controlsignal 120 b from the driver control circuit 116 of FIG. 1 adjusts adrive strength parameter affecting the drive strength of thedifferential driver 404.

In one embodiment, the differential drivers 400 and 404 may be currentmode drivers, and the drive strength control signals 120 may controltheir drive strength. A current mode driver typically usesNorton-equivalent parallel termination and generally containstransistors that operate in the saturation region of the I-V curve.Current mode drivers are used in high performance serial links, e.g.,communication channel 140, as a result of the high output common-modekeeping the current source saturated. The driver control circuit 116,illustrated in FIG. 1, receives the distortion information 112 andgenerates the two separate drive strength control signals 120 a and 120b for controlling the currents of the current sources in thedifferential drivers 400 and 404. As the drive strength parameterschange, the driver control circuit 116 may cause one current source toincrease its current while causing the other current source to decreaseits current. This causes output voltage of the differential driver 400to change.

In one embodiment, the drive strengths of drivers 400 and 404 may beadjusted in a manner that complies with a peak power constraintrequirement for the driver circuit 128. The peak power constraintspecifies the amount of power a data transmission system can safelyconsume, and can influence reliability. In some embodiments, the peakpower may be limited by the peak voltage rating of the data transmissionwires 148 and their characteristic impedance. The peak power constraintspecification may include the length of a duty cycle and average powerconstraint.

In an embodiment, the differential drivers 400 and 404 may be voltagemode drivers, and drive strength control signals 120 may control theirdrive strength. A voltage-mode driver may have multiple drive segmentsand the number of active drive segments that are turned on determinesthe drive strength of the driver.

The differential outputs of the drivers 400 and 404 are combined togenerate a combined differential signal 132 including two complementarysignals. The outputs of the drivers 400 and 404 are cross-wired suchthat the combined differential signal 132 is approximately equal to thepositive differential input pair (P) minus the negative differentialinput pair (N).

The MSB driver 400 has substantially two times the drive strength of theLSB driver 404. The drive strength of the MSB driver 400 can be adjustedindependently of the drive strength of the LSB driver 404. If there areinequalities between the voltage differences 358A, 358B, 358C as shownin FIG. 3, the ratio between the drive strength of the MSB driver 400and the drive strength of the LSB driver 404 can be adjusted to minimizethese inequalities. The driver control circuit 116 determines, from thedistortion information 162, how to adjust the ratio of the drivestrength of the MSB driver 400 to the LSB driver 404. In the case of aPAM-4 driver, if the distortion information 162 indicates that leveldifference 358A and 358C (which are equal by construction) are smallerthan level difference 358B, this means the drive strength of the LSBdriver 404 is not high enough when compared to the drive strength of MSBdriver 400. As a result, the driver control circuit 116 may increase thedrive strength of LSB driver 404 while reducing the drive strength ofMSB driver 400. Conversely, if the distortion information 162 indicatesthat level difference 358A and 358C are larger than level difference358B, this means the drive strength of the LSB driver 404 is too highwhen compared to the drive strength of MSB driver 400. As a result, thedriver control circuit 116 may reduce the drive strength of LSB driver404 while increasing the drive strength of MSB driver 400. Thecorrection loop may operate in a “bang-bang” fashion, in which a fixedsmall correction, in the direction dictated by the distortioninformation 162, is applied in each step.

Distortion Detection Circuit Using Eye Scanning

FIG. 5 is an illustration of an example distortion detection circuit 158of the PAM-N receiving device 154 of FIG. 1, according to an embodimentof the present disclosure. In this embodiment, eye scanning is used todetect distortions in the PAM-N signal 176. The distortion detectioncircuit 158 includes an eye scanning circuit 504 to detect vertical eyeopenings for the PAM-N signal 176 and to generate vertical eye openinginformation 558 describing the height of the vertical eye openings. Thedistortion information is generated from the vertical eye openinginformation 558 as illustrated and described above with reference toFIG. 3.

The interior region of the eye patterns shown in FIG. 3 is called theeye opening. There are three eye openings in the eye diagram 350 in FIG.3. The distortion detection circuit 158 measures the vertical height ofthe three eye openings at a particular time within the data eye diagram350. The relationship between the three eye openings may be equivalentto the relationship between level differences 358A, 358B and 358C. Inone embodiment, the eye scanning circuit can determine the vertical eyeopenings with a comparator that compares the PAM-N signal 176 to areference voltage. The reference voltage is adjusted while monitoringthe output of the comparator until the output of the comparatorindicates that the vertical boundary of a data eye opening is reached.The vertical eye opening information 558 can be determined for all dataeyes of the data eye 350, or it can be determined for only a subset ofthe data eyes such as the intermediate data eye between symbols 01 and10 and the upper data eye between symbols 10 and 11.

The distortion calculation circuit 508 calculates an amount ofdistortion from the vertical eye opening information 558 and thenoutputs this calculation as the distortion information 162. For example,the upper or lower data eye opening (the eye between the 10 and 11 datalines or between the 00 and 01 data lines) may be compared to the middledata eye opening (the eye between the 01 and 10 data lines). If theupper or lower eye openings are larger than the middle eye opening, thisis an indication that the level differences 358A and 358C are largerthen the level difference 358B. Conversely, if the upper and lower eyeopenings are smaller than the middle eye opening, this is an indicationthat that the level differences 358A and 358C are smaller then the leveldifference 358B. This information is output as distortion information162. To the first order, the ratio of the upper and lower eyes to themiddle eye is also the same as the ratio of the level differences 358Aand 358C to the level difference 358B. This information my also beincluded in the distortion information 162.

Detecting Distortion in a PAM-N Signal Based on ISI

FIG. 6 illustrates an example method of detecting distortion in thePAM-N signal 176 based on estimation of intersymbol interference (ISI),according to an embodiment of the present disclosure. The method can beperformed by the distortion detection circuit 158. ISI is a form ofdistortion of the signal 176 in which one symbol interferes withsubsequent symbols, making the communication less reliable. ISI may becaused by multipath propagation or the frequency response of a channelcausing successive symbols to “blur” together.

FIG. 6 illustrates the relationship between the data symbols (MSB 432and LSB 436) to the ideal analog voltage levels 304 of the PAM-N signal176. For the symbol 00, the MSB 432 is 0, the LSB 436 is 0, and theanalog voltage level 304 of the PAM-N signal 176 should ideally be −3 V.For the symbol 01, the MSB 432 is 0, the LSB 436 is 1, and the analogvoltage level 304 of the PAM-N signal 176 should ideally be −1 V. Forthe symbol 10, the MSB 432 is 1, the LSB 436 is 0, and the analogvoltage level 304 of the PAM-N signal 176 should ideally be 1 V. For thesymbol 11, the MSB 432 is 1, the LSB 436 is 1, and the analog voltagelevel 304 of the PAM-N signal 176 should ideally be 3 V. The differencebetween the signal levels 304 for the digital values 00 and 01, denotedby 308C in FIG. 3, should ideally be equal to (−1−−3) V or 2 V. Thedifference between the signal levels 304 for the digital values 01 and10, denoted by 308B in FIG. 3, should ideally be equal to (−1−1) V or 2V. The difference between the signal levels 304 for the digital values10 and 11, denoted by 308A in FIG. 3, should ideally be equal to (3−1) Vor 2 V. Therefore, the three level differences 308 should be equal toeach other. When there is nonlinearity distortion in the PAM-N signal176, e.g., in the eye diagram 350 in FIG. 3, the three level differences358 do not equal each other. In this event, the level differences 358will not equal 2 V each.

In the embodiment illustrated in FIG. 6, the distortion detectioncircuit 158 of FIG. 1 detects the distortion caused by the inequality inthe voltage differences 358 between the N signal levels 308 byidentifying a first amount of ISI during transitions from a first symbolof the N symbols, and identifying a second amount of ISI duringtransitions from a second symbol of the N symbols, wherein thedistortion information is generated from the first amount of ISI and thesecond amount of ISI.

In the transition 608 illustrated in FIG. 6, the PAM-N signal 176transitions from the symbol 00 at time 616 to the symbol 11 (the firstsymbol) to the symbol 00 (a third symbol) at time 628. The second changefrom the symbol 11 to the symbol 00 is the transition of interest here,especially at time 620. The ideal signal level 304 for symbol 11 is 3 V,and for symbol 00 is −3 V. Therefore, the transition 608 should have anideal output voltage swing of (3−−3) V or 6 V. Due to ISI from theprevious symbol 11, when transitioning from the symbol 11 to the symbol00, the signal experiences a first amount of ISI 624 at time 620 insteadof smoothly transitioning to the symbol 00.

In the transition 612, the PAM-N signal 176 transitions from the symbol00 at time 632 to a symbol 01 (the second symbol) to the symbol 00 (afourth symbol) at time 644. The second change from the symbol 01 to thesymbol 00 is the transition of interest here, especially at time 640.The ideal signal level 304 for symbol 01 is −1 V, and for symbol 00 is−3 V. Therefore, the transition 608 should have an ideal output voltageswing of (−1−−3) V or 2 V, which is one third of the voltage swingexperienced in transition 608. Due to ISI from the previous symbol 01,when transitioning from the symbol 01 to the symbol 00, the signalexperiences a second amount of ISI 636 at time 640 instead of smoothlytransitioning to the symbol 00.

The distortion detection circuit 158 may determine the distortion in thesignal levels from the ratio of the amount of ISI 624 to the amount ofISI 636. The amount of ISI 624 is determined from the transition 608 ofsymbol 11 at 3 V to symbol 00 at −3 V. This represents a 6 V outputvoltage swing in the PAM-N signal level 304. The amount of ISI 636 isdetermined from the transition 612 of symbol 01 at −1 V to symbol 00 at−3 V. This represents a 2 V output voltage swing in the PAM-N signallevel 304. Because the ratio of swing in voltage levels of the twotransitions is 6:2 or 3, therefore, the ratio between the second amountof ISI 636 and the first amount of ISI 624 should equal 1/3. Inembodiments, the two raw ISI values may be generated by the distortiondetection circuit 158. In embodiments, the determined ratio of the twoamounts of ISI may be generated by the distortion detection circuit 158.In embodiments, the deviation of the determined ISI ratio from the idealISI ratio (1/3) is output as distortion information 162.

In the case of the PAM-4 signal of FIG. 6, if the ISI value 636 issmaller than one third of the ISI value 624, this is an indication thatthe level difference 358C is smaller than the level difference 358B.Conversely, if the ISI value 636 is larger than one third of the ISIvalue 624, this is an indication that the level difference 358C islarger than the level difference 358B. The distortion detection circuit158 may generate distortion information 162 using this information. TheISI values 636 and 624 maybe calculated using DFE error level adaptationwith the addition of data filtering. Data filtering can refer toanalyzing the data decisions (e.g., data symbols 184) to identify thepresence of a target data symbol sequence and then obtaining the ISIvalues when those target data symbol sequences are present.

In practice different combinations of the first, second, third, andfourth symbol may be used to calibrate the receiver/transmitter based onthe analog voltage differences between symbols. For example, atransition from 10 (1 V) to 00 (−3 V), i.e., an output swing of 4 V,should experience double the ISI that a transition from 01 (−1 V) to 00(−3 V), i.e., an output swing of 2 V, does. Therefore, the distortion isdetected by identifying the first amount of ISI 624 during transitionsfrom a first symbol 11 of the N symbols, and identifying a second amountof ISI 636 during transitions from a second symbol 01 of the N symbols,wherein the distortion information is generated from the first amount ofISI 624 and the second amount of ISI 636. Further, the distortiondetection circuit 158 detects the distortion caused by the inequality inthe voltage differences 358 between the N signal levels 308 byidentifying the first amount of ISI 624 during transitions to a thirdsymbol 00 of the N symbols, and identifying the second amount of ISI 636during transitions to a fourth symbol 00 of the N symbols.

The driver circuit 128 of FIG. 1 may generate the PAM-N signal 132 usinga predetermined pattern specifically identified for use in calibratingthe receiver/transmitter system. In an embodiment, a predeterminedpattern may be used in a confirmatory way to verify that a givensequence of digital values produces expected results duringtransmission. The predetermined pattern may be produced in a focused orsystematic way by using repetitions of certain transitions, or by usinghigh-volume randomized automated tests to generate the PAM-N signal 132using pseudorandom or scrambled patterns. A pseudorandom patternexhibits statistical randomness while being generated by an entirelydeterministic causal process. The benefit of this approach is that it iseasier to produce than a genuinely random pattern, and has the benefitthat it can be used again and again to produce the same results, whichis useful for testing and calibrating the receiver/transmitter. In anembodiment, a scrambled pattern such as that corresponding to ascrambled idle pattern may be used.

In an embodiment, the distortion detection circuit 158 detects when apredetermined pattern is present in the PAM-N signal 176 and detects thedistortion caused by inequality in the voltage differences 358 betweenthe N signal levels 354 responsive to the predetermined pattern beingpresent. For example, each time the distortion detection circuit 158detects a transition from symbol values 11 to 00, it may store theamount of ISI 624 detected. Each time the distortion detection circuit158 detects a transition from symbol values 01 to 00, it may store theamount of ISI 636 detected. Over an interval of time, the distortiondetection circuit 158 determines the amount of distortion by aggregatingthe ISI values in response to the predetermined pattern of transitions.

Detecting Distortion in a PAM-N Signal Based on DFE Error LevelAdaptation

FIG. 7 illustrates an example method of detecting distortion in a PAM-Nsignal based on DFE error level adaptation, according to an embodimentof the present disclosure. In this embodiment, the PAM-N receivingdevice 154 of FIG. 1 includes a DFE 700. The DFE 700, PAM-N slicer 730,and summer circuit 716 may be parts of the decision circuit 180. A DFEis a nonlinear equalizer that uses the feedback of detected symbols 184to produce an estimate of the channel output. The DFE 700 is fed withdetected symbols 184 and produces outputs 722, 724, and 726, whichtypically are subtracted from the PAM-N output signal 132 of thereceiving circuit 172 of FIG. 1.

The DFE 700 includes DFE summing circuit 716, decision slicer circuit730, and DFE tap DACs circuit 736. The DFE tap DACs circuit 736 includesmultiple delay circuits, such as the three delay circuits 702, 704 and706, to delay the data symbols 184. The output of delay circuit 702 isone feedback tap, the output of delay circuit 704 is another feedbacktap, and the output of delay circuit 706 is another feedback tap. TheDFE 700 compensates for ISI generated by previous received datasymbols—Data(T1), Data(T2), and Data(T3). Each DFE weight W1, W2, and W3is an estimation of the ISI contribution from the corresponding tap.

Tap weighting circuits 742, 744 and 746 weight the delayed data of eachfeedback tap by a tap weight W1, W2 and W3 and output weighted feedbacksignals 722, 724 and 726. In one embodiment, weighting circuits 742, 744and 746 may be gain adjustable digital to analog converters (DAC) thatconvert the delayed data at each feedback tap into analog voltages.Summing circuit 716 subtracts the weighted feedback signals 722, 724 and726 from the PAM-N signal 132. The summing circuit 716 then outputs adecision equalized signal 718 resulting from the combination of theweighted feedback signals 722, 724 and 726 and PAM-N signal 132.

The PAM-N decision slicer 730 makes a decision on the data symbolsrepresented by the equalized signal 718, thereby generating recovereddata symbols 184. The recovered data symbols 184 are PAM-N symbols. Thedata symbols 184 are passed on to other circuit stages (not shown) thatuse the data symbols 184. In one embodiment, for PAM-4 symbols, thePAM-N decision slicer 730 includes three separate slicing circuits thatcompare the equalized signal 718 to three separate decision referencevoltages.

The DFE adaptation engine 172 is responsible for implementing anadaption algorithm that tunes the tap weights W1, W2, W3 of the DFE 700.The DFE adaption engine 172 can generate an error reference signal 710.The Error comparator 708 compares the decision equalized signal 718 tothe error reference signal 710. Error comparator 708 outputs an errorsignal 750 indicating whether the differential voltage level of thedecision equalized signal 718 is greater than or less than the errorreference signal 710. For example, error signal 750 can include a binaryvalue of 1 or 0 depending on the comparison. The error signal 750 istransmitted to the DFE adaptation engine 172, which iteratively adjuststhe error reference signal 710 and the tap weights W1, W2 and W3 basedon the error signal 750 using an adaptation algorithm. The DFEadaptation engine 172 may also receive and use the data symbols 184 anduse the during the adaptation process.

During DFE error level adaptation, the error reference signal 710 istypically set by targeting one symbol at a time (e.g. 10, 11), and thenadjusting the error reference signal 710 to a threshold error levelwhile that symbol is being targeted. The DFE error level adaptationprocess typically results in the error reference signal 710 being set toa threshold error level that is close to the edge of a data eye. Forexample, referring to the data eye diagram 350 of FIG. 3, the errorreference signal 710 can be set to 3 volts when targeting a “11” symbol,can be set to 1 volts when targeting a “10” symbol, can be set to −1volts when targeting a “01” symbol and can be set to −3 volts whentargeting a “00” symbol.

The error reference signal 710 is also transmitted by the DFE adaptationengine 172 to the distortion detection circuit 158, which generates thedistortion information 162.

The distortion detection circuit 158 generates the distortioninformation 162 by determining a first threshold error level of theerror reference signal 710 used to generate error information 750 foradaptation of tap weights W1, W2 and W3 of the DFE 700 when targeting afirst symbol of the N symbols, e.g., digital value 11. The distortiondetection circuit 158 determines a second threshold error level of theerror reference signal 710 used to generate error information 750 foradaptation of the tap weights W1, W2 and W3 of the DFE 700 whentargeting a second symbol of the N symbols, e.g., 10. The error levelsused for DFE error level adaptation are correlated to the leveldifferences 358A, 358B and 358C and can be used as an indication ofdistortion corresponding to inequality in voltage differences. Forexample, ideally, the error level when targeting a “11” symbol should bethree times the error level when targeting a “10” symbol. However, ifthere is distortion, as shown in the data eye diagram 350 of FIG. 3,then the two error levels will not be three times each other.

The distortion information 162 is generated from the first thresholderror level and the second threshold error level. In one embodiment, thedistortion information 162 can be generated by calculating a ratio ofthe threshold error level for the “11” symbol to the ratio of thethreshold error level for the “10” symbol. If the ratio of the thresholderror level for the “11” symbol to the threshold error level for the“10” signal is more than three, this is in an indication that the leveldifference 358A is larger than the level difference 358B. Conversely, ifthe ratio of the threshold error level for the “11” symbol to thethreshold error level for the “10” signal is less than three, this is inan indication that the level difference 358A is smaller than the leveldifference 358B. This information is used to output distortioninformation 162. Error levels targeting other symbols can be used to getinformation on the three level difference 358A, 358B, and 358C. Errorlevels are obtained in the process of conventional DFE error leveladaptation in conjunction with proper data filtering.

A Method of Receiver/Transmitter Co-Calibration of Voltage Levels inPAM-N Links

FIG. 8 is an example flowchart illustrating a method ofreceiver/transmitter co-calibration of voltage levels in PAM-N links,according to an embodiment of the present disclosure. In someembodiments, the processes may have different and/or additional stepsthan those described in conjunction with FIG. 8. Steps of the processesmay be performed in different orders than the order described inconjunction with FIG. 8. Some steps may be executed in parallel.Alternatively, some of the steps may be executed in parallel and somesteps executed sequentially. Alternatively, some steps may execute in apipelined fashion such that execution of a step is started before theexecution of a previous step.

The PAM-N transmitting device 104 of FIG. 1, transmits 800 via thedriver circuit 128, a PAM-N signal 132 via a communication channel 140.Here, N is greater than 2 and the PAM-N 132 signal has N signal levelscorresponding to N symbols, e.g., 000, 001, 010, etc. The PAM-Nreceiving device, receives 804 via the receiver interface circuit 172,the PAM-N signal 176 (after voltage offset correction, etc., by the AFE200). The distortion detection circuit 158 detects 808 the non-lineardistortion corresponding to inequalities in voltage differences 358between the N signal levels 354 within the PAM-N signal 176 andgenerates distortion information indicative of the level of thedistortion.

The PAM-N receiving device 154 transmits 812, via the driver circuit 166to the PAM-N transmitting device 104, the distortion information 162indicative of a level of the distortion. The PAM-N transmitting device104 receives, via the receiver interface circuit 108, the distortioninformation 162. The PAM-N transmitting device 104 adjusts 816, via thedriver control circuit 116, one or more drive strength parameters of thedriver circuit 128 of the PAM-N transmitting device 104 based on thedistortion information 112.

In one embodiment, a representation of circuits within thereceiver/transmitter calibration architecture may be stored as data in anon-transitory computer-readable medium (e.g. hard disk drive, flashdrive, optical drive). These representations may in the form of, forexample, behavioral level descriptions, register transfer leveldescriptions, logic component level descriptions, transistor leveldescriptions or layout geometry-level descriptions.

Upon reading this disclosure, those of skill in the art may appreciatestill additional alternative designs for a system that includes a PAM-Ntransmitting device and a PAM-N receiving device. Thus, while particularembodiments and applications of the present disclosure have beenillustrated and described, it is to be understood that the disclosure isnot limited to the precise construction and components disclosed herein.Various modifications, changes and variations which may be apparent tothose skilled in the art may be made in the arrangement, operation anddetails of the method and apparatus of the present disclosure hereinwithout departing from the spirit and scope of the disclosure as definedin the appended claims.

What is claimed is:
 1. A PAM-N receiving device, comprising: a receiverinterface circuit to receive a PAM-N signal from a PAM-N transmittingdevice via a communication channel, wherein N is greater than 2 and thePAM-N signal has N signal levels for N symbols; a circuit to generatedistortion information indicative of a level of distortion correspondingto inequalities in voltage differences between the N signal levels; anda driver circuit to transmit the distortion information indicative ofthe level of the distortion to the PAM-N transmitting device.
 2. ThePAM-N receiving device of claim 1, further comprising: a decisionfeedback equalizer, wherein the distortion information is generated by:determining a first threshold error level used to generate errorinformation for adaptation of tap weights of the decision feedbackequalizer when targeting a first symbol of the N symbols, anddetermining a second threshold error level used to generate errorinformation for adaptation of the tap weights of the decision feedbackequalizer when targeting a second symbol of the N symbols, wherein thedistortion information is generated from the first threshold error leveland the second threshold error level.
 3. The PAM-N receiving device ofclaim 1, wherein the distortion information is generated by: identifyinga first amount of inter symbol interference (ISI) during transitionsfrom a first symbol of the N symbols; and identifying a second amount ofISI during transitions from a second symbol of the N symbols, whereinthe distortion information is generated from the first amount of ISI andthe second amount of ISI.
 4. The PAM-N receiving device of claim 3,wherein the circuit: identifies the first amount of ISI duringtransitions from the first symbol to a third symbol of the N symbols;and identifies the second amount of ISI during transitions from thesecond symbol to a fourth symbol of the N symbols.
 5. The PAM-Nreceiving device of claim 1, wherein the circuit comprises: an eyescanning circuit to detect a plurality of vertical eye openings for thePAM-N signal, wherein the distortion information is generated from thevertical eye openings.
 6. The PAM-N receiving device of claim 1,wherein: the circuit detects when a predetermined pattern is present inthe PAM-N signal and generates the distortion information responsive tothe predetermined pattern being present.
 7. The PAM-N receiving deviceof claim 1, wherein: the receiver interface circuit comprises an analogfront end circuit that applies analog signal conditioning to the PAM-Nsignal; and the circuit generates the distortion information indicativeof the level of distortion corresponding to the inequalities in thevoltage differences between the N signal levels in the PAM-N signal atan output of the analog front end circuit.
 8. A PAM-N transmittingdevice, comprising: a driver circuit to transmit a PAM-N signal via acommunication channel to a PAM-N receiving device, wherein N is greaterthan 2, and the PAM-N signal has N signal levels for N symbols; areceiver interface circuit to receive distortion information indicativeof a level of distortion corresponding to inequalities in voltagedifferences between the N signal levels at the PAM-N receiving device;and a driver control circuit to adjust one or more drive strengthparameters of the driver circuit based on the distortion information. 9.The PAM-N transmitting device of claim 8, wherein: the driver circuitgenerates the PAM-N signal using a predetermined pattern.
 10. The PAM-Ntransmitting device of claim 8, wherein: the driver circuit generatesthe PAM-N signal using a pseudorandom or scrambled pattern.
 11. ThePAM-N transmitting device of claim 8, wherein the driver circuitincludes: a first driver corresponding to a first symbol bit; and asecond driver corresponding to a second symbol bit, wherein the drivercontrol circuit adjusts the one or more drive strength parameters basedon the distortion information such that a ratio of a first drivestrength of the first driver to a second drive strength of the seconddriver is adjusted.
 12. The PAM-N transmitting device of claim 8,wherein the driver control circuit adjusts the one or more drivestrength parameters to minimize the inequalities in the voltagedifferences.
 13. The PAM-N transmitting device of claim 8, wherein thedriver control circuit adjusts the one or more drive strength parametersto match a peak power constraint of the driver circuit.
 14. A method,comprising: transmitting, by a driver circuit of a PAM-N transmittingdevice, a PAM-N signal via a communication channel, wherein N is greaterthan 2, and the PAM-N signal has N signal levels corresponding to Nsymbols; receiving, by a PAM-N receiving device, the PAM-N signal;generating, by the PAM-N receiving device, distortion informationindicative of a level of distortion corresponding to inequalities involtage differences between the N signal levels; transmitting, by thePAM-N receiving device to the PAM-N transmitting device, the distortioninformation indicative of the level of the distortion; receiving, by thePAM-N transmitting device, the distortion information; and adjusting, bythe PAM-N transmitting device, one or more drive strength parameters ofthe driver circuit of the PAM-N transmitting device based on thedistortion information.
 15. The method of claim 14, wherein thegenerating of the distortion information comprises: determining a firstthreshold error level used to generate error information for adaptationof tap weights of a decision feedback equalizer when targeting a firstsymbol of the N symbols; and determining a second threshold error levelused to generate error information for adaptation of the tap weights ofthe decision feedback equalizer when targeting a second symbol of the Nsymbols, wherein the distortion information is generated from the firstthreshold error level and the second threshold error level.
 16. Themethod of claim 14, wherein the generating of the distortion informationcomprises: identifying a first amount of ISI during transitions from afirst symbol of the N symbols; and identifying a second amount of ISIduring transitions from a second symbol of the N symbols, wherein thedistortion information is generated from the first amount of ISI and thesecond amount of ISI.
 17. The method of claim 16, wherein the generatingof the distortion information comprises: identifying the first amount ofISI during transitions from the first symbol to a third symbol of the Nsymbols; and identifying the second amount of ISI during transitionsfrom the second symbol to a fourth symbol of the N symbols.
 18. Themethod of claim 14, wherein the generating of the distortion informationcomprises: detecting a plurality of vertical eye openings for the PAM-Nsignal, wherein the distortion information is generated from thevertical eye openings.
 19. The method of claim 14, further comprising:detecting a predetermined pattern being present in the PAM-N signal; andgenerating the distortion information responsive to the predeterminedpattern being present.
 20. The method of claim 14, further comprising:applying, by an analog front end circuit, analog signal conditioning tothe PAM-N signal; wherein the distortion information is generated toindicate a level of distortion corresponding to the inequalities in thevoltage differences between the N signal levels in the PAM-N signal atan output of the analog front end circuit.